In recent years, communication organizations have been studying NGN (Next Generation Networks). In the NGN architecture, a NACF (Network Attachment Control Function) manages the edge nodes of a core network, and the correspondence of user terminals connected to the edge nodes (for example, “Telecommunications and Internet converged Services and Protocols for Advanced-Networking (TISPAN); NGN Functional Architecture Release 1”, ETSI ES 282 004 V1.1.1, Section 5.3.2; [online], searched on Oct. 4, 2007, Internet.
In NGN, an IMS (IP Multimedia Subsystem) is applied to session control.
IMS is being designed by the 3GPP (3rd Generation Partnership Project) and 3GPP2 (3rd Generation Partnership Project 2) that aim to standardize 3rd generation mobile radio communication networks. 3GPP2 manages correspondence information about user terminals and PDSN (Packet Data Serving Nodes) that are edge nodes, and node AAA (Authentication, Authorization, and Accounting) is performed to authenticate connection of the user terminal with the PDSN. When a presence function is provided on IMS, a presence server can acquire the correspondence information about the user terminals and PDSN. For example, the presence function can grasp the current status of a user on the network. Specifically, the presence server can acquire correspondence information about user terminals and the PDSN from the Authentication node AAA via a node PNA (Presence Network Agent) that relays presence information (for example, 3GPP2 X. P0027-004-0 v1.0 V&V Network Presence Section 4, [online], searched on Oct. 4, 2007, Internet.
At the same time, standardization of IPTV (Internet Protocol TeleVision) that is now attracting attention as a promising NGN service, is-being promoted by the ITU-T (International Telecommunication Union Telecommunication Standardization Sector) FG-IPTV (Focus Group-IPTV). In VOD (Video On Demand) that is one type of IPTV service, a content distribution server distributes image content by unicast to user terminals. In VOD, there is a system that streams content, and a system that downloads content to the user terminal.
An example of content playback control protocol is RTSP (Real-time Streaming Protocol), and an example of streaming data transfer protocol is RTP (Real Time Protocol). Both RTSP and RTP are protocols specified by the IETF (Internet Engineering Task Force), and are generally used with a streaming service.
To distribute content efficiently at high speed, CDN (Contents Delivery Network) is usually used. In CDN, part or all of the content held by an original content server is saved on a cache server, and then distributed. A user terminal accesses the cache server or a mirror server near the user terminal instead of accessing the original content server, and receives the desired content. In prior art CDN technology, the content saved on the cache server is deleted at suitable intervals according to the content, and re-cache is performed upon a subsequent request (for example, Hiroshi Kobayashi, Hiroshi Ezaki, “Internet Overview”, published by Kyoritsu Shuppan, Co., Ltd., Jan. 25, 2002, page 111).
As an example of a content delivery system which allows plural users to view the same content, JP-A-2005-269411 discloses that a device on the server side manages playback stop position information for each user terminal. When the user terminal starts the VOD service again, the playback stop position information is used in order to start playback from the position where viewing stopped on the immediately preceding occasion.